The instant invention relates to a ceramic filter with an open-celled foam structure having at least one afflux plane for filtering molten metals.
Filtration of metallic melts with ceramic foam filters is generally known in the field of non-iron metals. Particularly in the production of semifinished materials, a high degree of purity is necessary since layers of very small thickness are produced in the succeeding operational steps. Therefore, solid or molten impurities must be removed from the melt during production since they would otherwise lead to a considerable deterioration in quality. In this connection, ceramic filters, especially those based upon foam structures, have been successfully used for several years.
For example, U.S. Pat. Nos. 3,893,917; 4,265,659; and 4,343,704, all of which are incorporated herein by reference, disclose ceramic foam filters for filtering molten metals, particularly molten aluminum. These ceramic foam filters include a foam structure which is prepared from an open cell, hydrophilic flexible foam material having a plurality of interconnected voids surrounded by a web of ceramic material. Typical foam materials which may be used include the polymeric foams such as polyurethane foams and the cellulosic foams. The foam structure is impregnated with a ceramic material which is able to withstand chemical attack by the particular molten metal. The ceramic material should also have sufficient structural and mechanical strength to withstand the required elevated temperature conditions. Typical ceramic materials which may be employed include alumina (Al.sub.2 O.sub.3), chromia (Cr.sub.2 O.sub.3), zirconia (ZrO.sub.2), magnesia (MgO), titanium dioxide (TiO.sub.2), silica (SiO.sub.2), and mixtures thereof.
While ceramic filters and filtration systems using such ceramic filters are known in the art for filtering various kinds of molten metals, special problems are encountered in filtering molten iron.
When liquid iron passes through a ceramic filter, coarse impurities such as sand grains, small dross particles and non-dissolved seeding sites are removed from the melt on the entry side of the filter. Fine impurities, such as oxidation films and similar materials, deposit increasingly in the pores located within the filter. Thus, the coarse impurities are completely filtered out but the finely dispersed impurities are only partly removed. This problem may be alleviated somewhat by the use of fine-pored filters.
In iron melts containing magnesium, however, the numerous MgO and MgS slags present lead to a premature clogging of the filter. For this reason, the filter for manufacturing for example cast pieces of GGG, also known as nodular cast iron, must not be too fine-grained.
Furthermore, in casting iron melts which have a much higher temperature than aluminum melts, special requirements are placed on the filter in use. On the one hand, the filter must withstand the high temperatures until the mold is filled while maintaining a high rate of filtering. On the other hand, the filtration times must be adjusted to correspond to the piece to be cast. These requirements can be met by adjusting the specific filter surface, the pressure loss within the filter, and other parameters.
It is therefore an object of the present invention to provide a ceramic filter which is suitable for filtering molten cast iron.
It is also an object of the present invention to provide a method for filtering molten cast iron by means of a ceramic filter.